Conventionally, an organic EL display is known. The organic EL display includes a driving thin film transistor (“TFT”) for driving the organic EL element with a current, and in the organic EL display, an amount of current of the driving TFT is controlled according to image data.
Due to reasons such as a problem in manufacturing or degradation with elapse of time, the Vth of the driving TFT or a slope (μ) of the V-I characteristic can vary, resulting in brightness unevenness. In order to correct the unevenness, there are cases in which a predetermined value is added to the image data for driving the pixels to correct Vth (offset correction) and in which the image data is multiplied by a predetermined value to correct μ (gain correction).
FIG. 3 shows a method of calculating correction data and FIG. 4 is a block diagram of a correction circuit. First, voltage-current characteristics of a number of pixels are measured in order to determine a curve of the V-I characteristic of standard pixels in the panel. A function f(x) is determined assuming that this curve is a curve represented by an equation of I=f(a(V−b)). Assuming that a total of pixels of the panel is represented by f(x) and the variation in the characteristics is due to differences in the coefficients a and b, it is possible to determine the coefficients a and b of the pixels by, for example, measuring pixel currents corresponding to two or more input voltage levels.
When the V-I characteristic of a pixel p is represented by I=f(a′(V−b′), it is possible to determine, based on the coefficients a and b for an average pixel which are already determined, offset=k(b′−ab/a′) and gain=a/a′ with the coefficient k being a coefficient of the D/A conversion, and correction can be executed by multiplying the image data by the determined gain and adding the determined offset.
More specifically, as shown in FIG. 4, for each of an R signal, a G signal, and a B signal which are image data, γ-corrected image data is obtained in a γ lookup table (γLUT) 10 for obtaining a linear relationship between input pixel data and pixel current. The γ-corrected image data is multiplied by the correction gain, gain, in a multiplier 12 and a correction offset, offset, is added in an adder 14.
An image signal (R, G, B) for which unevenness is corrected is supplied to a display panel 18 through a data driver 16 having a data latch and a D/A converter, and is displayed in the display panel 18. A gate driver 20 is connected to the display panel 18, and the gate driver 20 controls to which line of the display panel 18 the image data is supplied.
A timing signal generating unit 22 generates various timing signals based on a pixel clock, a horizontal synchronization signal, and a vertical synchronization signal, and also generates an address of a memory 24. The memory 24 includes a RAM which can be read and written quickly, and, when the power supply is started up, correction data (gain, offset) are sent from an external nonvolatile memory or the like through a correction data transferring circuit 26, and is stored in the memory 24. The timing signal generating unit 22 generates, in correspondence with the image data of each pixel, an address at which the correction data for that pixel is stored, the correction data for each pixel is read from the memory 24, and the correction data are supplied to the multiplier 12 and the adder 14 through a correction gain generating circuit 28 and a correction offset generating circuit 30. The correction gain generating circuit 28, the correction offset generating circuit 30, the multiplier 12, and the adder 14 form a correction calculating unit 29.
In this manner, the unevenness can be significantly improved by calculating the γ-corrected signal data. Such an unevenness correction is described in, for example, JP 11-282420; U.S. Pat. No. 7,345,660, U.S. Patent Application Publication No. 2007/273701 and WO 2005/101360.
Some driver ICs for mobile devices have a built-in display RAM (display memory) which is called a graphic RAM, and do not require, once a static image is written to the display memory, transfer of the image signal from the outside unless the display image is to be changed. In FIG. 5, an image input signal interface 32 has functions to temporarily store an input image signal having 8 bits for each color in a display memory 34 and to send the stored image to the γLUT 10. In addition, for a signal which is continuously input in synchronization to the pixel clock, the signal can be sent to the γLUT 10 without any processing. The switching between these operations is achieved by a switching signal from a CPU or the like on the side of the system. In addition, in general, graphic functions are provided in which a line or a drawing is written on an image on the display memory 34, an image is scrolled, or an image is enlarged or reduced.
In a driver IC having such a display memory 34, although there are advantages that the unnecessary radiation from the data bus between the system-side circuit and the driver IC can be reduced, the power consumption by the data transfer can be reduced, and the load of the system-side circuit can be reduced, there also is a disadvantage in that the chip size is increased because of the large-capacity memory, and consequently the cost is increased. Currently, in general, a display memory is equipped in many cases in a panel for a portable phone having more frequent occurrences of occasions to display a still image, and the display memory is not equipped in a built-in panel for a monitor of a digital camera and a video camera which frequently displays an animated image also. Therefore, the determination of whether or not the display RAM is to be equipped cannot be generally made, and is currently comprehensively made according to the application.
When all pixels are to be corrected using the unevenness correction circuit, the correction data is required for each pixel, and consequently a memory for storing the data for the number of pixels of the panel is required. Currently, as shown in FIGS. 6 and 7, correction is executed by writing data of the unevenness in an external nonvolatile memory 36 at the time of shipping of the panel, reading all correction data to the memory (RAM) 24 in the driver IC when the power supply of the panel module is started up, and correcting with the use of the data in the memory 24. In this example configuration, the nonvolatile memory 36 is mounted on a flexible cable 38.
In the case of a display panel 18 with a large number of pixels, a large-capacity RAM would be required as the memory 24, which affects the chip size of the driver IC and also increases the cost. As described, the occupied percentage of the correction memory 24 in the size of the driver IC is significant, and for a panel having a smaller number of occurrences of the unevenness because of optimization of the TFT manufacturing process or devising of the pixel circuit, it is advantageous, in view of the cost, to not equip the circuit for unevenness correction using an external circuit in the panel.
On the other hand, some driver ICs for mobile devices are equipped with a display memory 34 which is a display RAM, and in this case also, the memory for the number of pixels of the panel is used. However, as described above, there are some applications that do not require the display memory.
In other words, in an ideal structure, for a panel having the same number of pixels and the same driving method, four types of driver ICs are provided including a driver IC having neither the unevenness correction function nor the display memory function, a driver IC having only the unevenness correction function, a driver IC having only the display memory function, and a driver IC having both the unevenness correction function and the display memory function, and the driver ICs are used differently according to the usage. However, in reality, it is difficult to develop four types of driver ICs in view of the development cost and the number of steps for development, and furthermore, because of the variation of the types of the components, cost reduction by mass production cannot be expected.